1. Field of the Invention
This invention relates generally to wireless communications and, more particularly, to the operation of a Radio Frequency (RF) transceiver within a component of a wireless communication system.
2. Description of the Related Art
The structure and operation of wireless communication systems are generally known. Examples of such wireless communication systems include cellular systems and wireless local area networks, among others. Equipment that is deployed in these communication systems is typically built to support standardized operations, i.e., operating standards. These operating standards prescribe particular carrier frequencies, modulation types, baud rates, physical layer frame structures, MAC layer operations, link layer operations, etc. By complying with these operating standards, equipment interoperability is achieved.
In a cellular system, a regulatory body typically licenses a frequency spectrum for a corresponding geographic area (service area) that is used by a licensed system operator to provide wireless service within the service area. Based upon the licensed spectrum and the operating standards employed for the service area, the system operator deploys a plurality of carrier frequencies (channels) within the frequency spectrum that support the subscribers' subscriber units within the service area. Typically, these channels are equally spaced across the licensed spectrum. The separation between adjacent carriers is defined by the operating standards and is selected to maximize the capacity supported within the licensed spectrum without excessive interference. In most cases, severe limitations are placed upon the amount of adjacent channel interference that maybe caused by transmissions on a particular channel.
In cellular systems, a plurality of base stations is distributed across the service area. Each base station services wireless communications within a respective cell. Each cell may be further subdivided into a plurality of sectors. In many cellular systems, e.g., Global System for Mobile Communications (GSM) cellular systems, each base station supports forward link communications (from the base station to subscriber units) on a first set of carrier frequencies, and reverse link communications (from subscriber units to the base station) on a second set of carrier frequencies. The first set and second set of carrier frequencies supported by the base station are a subset of all of the carriers within the licensed frequency spectrum. In most, if not all, cellular systems, carrier frequencies are reused so that interference between base stations using the same carrier frequencies is minimized and system capacity is increased. Typically, base stations using the same carrier frequencies are geographically separated so that minimal interference results.
Both base stations and subscriber units include RF transceivers. Radio frequency transceivers service the wireless links between the base stations and subscriber units. The RF transmitter receives a baseband signal from a baseband processor, converts the baseband signal to an RF signal, and couples the RF signal to an antenna for transmission. In most RF transmitters, because of well-known limitations, the baseband signal is first converted to an Intermediate Frequency (IF) signal and then the IF signal is converted to the RF signal. Similarly, the RF receiver receives an RF signal, down converts the RF signal to an IF signal and then converts the IF signal to a baseband signal. In other systems, the received RF signal is converted directly to a baseband signal.
In converting the RF signal to an IF signal, the RF signal is mixed with a signal having a specified frequency that is received from a local oscillator (LO). As used herein, “local oscillator” is a generic term used to describe a device that provides a fixed frequency to a mixer that the mixer mixes with a signal of interest, e.g., baseband signal, RF signal, or IF signal. Because the mixer's ability to down convert (or up convert) a received signal depends upon it receiving an accurate frequency signal from the local oscillator, many local oscillators are formed to be adjustable so as to adjust an output frequency to a number of supported RF channels and to account for variations due to temperature, process, manufacturing and other factors that may affect the precise frequency that is produced by the local oscillator.
RF transceivers typically operate across relatively large RF frequency range with tight operating tolerance. These operating requirements, as well as the process and environmental variations described above, place substantial burden on the PLL. As is known by one of average skill in the art, a voltage controlled oscillator (VCO) produces a signal having a frequency characteristic that is a function of an input voltage level of the VCO. A VCO is often formed within a typical PLL to produce signals having a large frequency range for a large range in input voltage levels to create a PLL that is sufficiently adjustable to account for the process and environmental variations. However, VCOs operate best, i.e., are most linear, within small frequency ranges. Thus, when a standard PLL is required to operate across a large frequency range, the VCO often operates in at least a non-linear region. In this/these non-linear regions of operation, the VCO produces significant phase noise which degrades the LO signal produced by the PLL and, resultantly, degrades the IF signal (or baseband signal) produced by the coupled mixer.
Because the variations due to the environmental and process factors can be quite large, however, it is often required that the local oscillator be adjustable over a large range. If, for example, a voltage controlled oscillator is often required to have an ability to provide frequencies of oscillation that correspond to a large input voltage range in order to provide the necessary frequency compensation to fine tune the voltage controlled oscillator to account for environmental variations, etc. Thus, increasing frequency range requirements for a PLL increases probably phase noise and decreases performance of a phase-locked loop that is being used to provide a signal having a specified frequency characteristic of the mixer. Thus, what is needed is a circuit that reduces the required gain level for the voltage controlled oscillator and therefore reduces any introduced phase noise while accurately producing a signal with a specified frequency.